Unicellular eukaryotes (Protista) often inhabit environments of extreme Ph. Such environments occur naturally and result from man-made activities. The diversity and physiology of protists living in such environments has been largely under studied. The proposed work will explore the molecular diversity of protists in extremely acidic and alkaline environments and consider the physiological adaptations which allow survival under pH extremes. This proposal aims to test the hypothesis that very diverse mechanisms regulate internal pH concentration in protists living in alkaline versus acidic pH environments. Comparisons of small-subunit ribosomal RNA sequences will be used to describe the molecular diversity of acidic and alkaline environments. The second component of the proposed work will include establishment of laboratory cultures of representative protists and physiological experiments aimed at distinguishing between different possible mechanisms of internal pH maintenance. The physiological experiments will utilize ion-selective vibrating probe technology to distinguish between different kinds of ion-transport mechanisms. These experiments will identify novel mechanisms of regulation which can then be pursued to a greater extent using molecular biological techniques. The proposed work will serve to broaden our understanding of biology in environments on the threshold of sustaining life.